Stapled α−helical peptides have emerged as a promising new modality for a wide range of therapeutic targets. Here, we report a potent and selective dual inhibitor of MDM2 and MDMX, ATSP-7041, which effectively activates the p53 pathway in tumors in vitro and in vivo. Specifically, ATSP-7041 binds both MDM2 and MDMX with nanomolar affinities, shows submicromolar cellular activities in cancer cell lines in the presence of serum, and demonstrates highly specific, on-target mechanism of action. A high resolution (1.7-Å) X-ray crystal structure reveals its molecular interactions with the target protein MDMX, including multiple contacts with key amino acids as well as a role for the hydrocarbon staple itself in target engagement. Most importantly, ATSP-7041 demonstrates robust p53-dependent tumor growth suppression in MDM2/MDMX-overexpressing xenograft cancer models, with a high correlation to on-target pharmacodynamic activity, and possesses favorable pharmacokinetic and tissue distribution properties. Overall, ATSP-7041 demonstrates in vitro and in vivo proofof-concept that stapled peptides can be developed as therapeutically relevant inhibitors of protein-protein interaction and may offer a viable modality for cancer therapy.T he human transcription factor protein p53 induces cell-cycle arrest and apoptosis in response to DNA damage and cellular stress and thereby plays a critical role in protecting cells from malignant transformation (1, 2). Inactivation of this guardian of the genome either by deletion or mutation or through overexpression of inhibitory proteins is the most common defect in human cancers (1, 2). Cancers that overexpress the inhibitory proteins MDM2 and MDMX also possess wild-type p53 (p53WT), and thus pharmacological disruption of the interactions between p53 and MDM2 and MDMX offers the opportunity to restore p53-dependent cell-cycle arrest and apoptosis in this important class of tumors (3-6).MDM2 negatively regulates p53 function through multiple mechanisms, including direct binding that masks the p53 transactivation domain, impairing nuclear import of the p53 protein, and ubiquitination and proteasomal degradation of the p53 protein (6, 7). Consequently, aberrant MDM2 overexpression and gene amplification contribute to accelerated cancer development and growth (1, 8). The other negative regulator, MDMX, possesses a similar p53-binding activity and also effectively inhibits p53 transcriptional activity. Amplification of MDMX is seen in many tumors, including melanoma, breast, head and neck, hepatocellular, and retinoblastoma, and, interestingly, amplification of MDMX appears to correlate with both p53WT status and an absence of MDM2 amplification (6, 9, 10). MDMX does not have the intrinsic E3 ubiquitin ligase activity of MDM2 and cannot affect p53 stability, but MDM2/MDMX heterodimers can increase ubiquitin ligase activity relative to the MDM2 monomer. Given these functional differences, MDM2 and MDMX are each unable to compensate for the loss of the other, and they regulate nonoverlapping fu...

Using an expanded genetic code, antibodies with site-specifically incorporated nonnative amino acids were produced in stable cell lines derived from a CHO cell line with titers over 1 g/L. Using anti-5T4 and anti-Her2 antibodies as model systems, site-specific antibody drug conjugates (NDCs) were produced, via oxime bond formation between ketones on the side chain of the incorporated nonnative amino acid and hydroxylamine functionalized monomethyl auristatin D with either protease-cleavable or noncleavable linkers. When noncleavable linkers were used, these conjugates were highly stable and displayed improved in vitro efficacy as well as in vivo efficacy and pharmacokinetic stability in rodent models relative to conventional antibody drug conjugates conjugated through either engineered surface-exposed or reduced interchain disulfide bond cysteine residues. The advantages of the oximebonded, site-specific NDCs were even more apparent when lowantigen-expressing (2+) target cell lines were used in the comparative studies. NDCs generated with protease-cleavable linkers demonstrated that the site of conjugation had a significant impact on the stability of these rationally designed prodrug linkers. In a single-dose rat toxicology study, a site-specific anti-Her2 NDC was well tolerated at dose levels up to 90 mg/kg. These experiments support the notion that chemically defined antibody conjugates can be synthesized in commercially relevant yields and can lead to antibody drug conjugates with improved properties relative to the heterogeneous conjugates formed by nonspecific chemical modification.A ntibody drug conjugates (ADCs) are emerging as a new class of anticancer therapeutics that combine the efficacy of small-molecule therapeutics with the targeting ability of an antibody (Ab) (1, 2). By combining these two components into a single molecular entity, highly cytotoxic small-molecule drugs (SMDs) can be delivered to cancerous target tissues, thereby enhancing efficacy while reducing the potential systemic toxic side effects of the SMD. Conventional ADCs are typically produced by conjugating the SMD to the Ab through the side chains of either surface-exposed lysines or free cysteines generated through reduction of interchain disulfide bonds (3, 4). Because antibodies contain many lysine and cysteine residues, conventional conjugation typically produces heterogeneous mixtures that present challenges with respect to analytical characterization and manufacturing. Furthermore, the individual constituents of these mixtures exhibit different pharmacology with respect to their pharmacokinetic, efficacy, and safety profiles, hindering a rational approach to optimizing this modality (5).Recently, it was reported that the pharmacological profile of ADCs may be improved by applying site-specific conjugation technologies that make use of surface-exposed cysteine residues engineered into antibodies (THIOMABS) that are then conjugated to the SMD, resulting in site-specifically conjugated ADCs (TDCs) with defined Ab-drug ratios. Rel...

Tigecycline is a novel glycylcycline antibiotic that possesses broad-spectrum activity against many clinically relevant species of bacterial pathogens. The mechanism of action of tigecycline was delineated using functional, biophysical, and molecular modeling experiments in this study. Functional assays showed that tigecycline specifically inhibits bacterial protein synthesis with potency 3-and 20-fold greater than that of minocycline and tetracycline, respectively. Biophysical analyses demonstrated that isolated ribosomes bind tigecycline, minocycline, and tetracycline with dissociation constant values of 10 ؊8 , 10 ؊7 , and >10 ؊6 M, respectively. A molecular model of tigecycline bound to the ribosome was generated with the aid of a 3.40-angstrom resolution X-ray diffraction structure of the 30S ribosomal subunit from Thermus thermophilus. This model places tigecycline in the A site of the 30S subunit and involves substantial interactions with residues of H34 of the ribosomal subunit. These interactions were not observed in a model of tetracycline binding. Modeling data were consistent with the biochemical and biophysical data generated in this and other recent studies and suggested that tigecycline binds to bacterial ribosomes in a novel way that allows it to overcome tetracycline resistance due to ribosomal protection.

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Mapping of the tryptase locus on chromosome 17 revealed a novel gene 2.3 kilobase 3' of the mouse mast cell protease (mMCP) 6 gene. This 3.7-kilobase gene encodes the first example of a protease in the tryptase family that contains a membrane-spanning segment located at its COOH terminus. Comparative structural studies indicated that the putative transmembrane tryptase (TMT) possesses a unique substrate-binding cleft. As assessed by RNA blot analyses, mTMT is expressed in mice in both strain- and tissue-dependent manners. Thus, different transcriptional and/or post-transcriptional mechanisms are used to control the expression of mTMT in vivo. Analysis of the corresponding tryptase locus in the human genome resulted in the isolation and characterization of the hTMT gene. The hTMT transcript is expressed in numerous tissues and is also translated. Analysis of the tryptase family of genes in mice and humans now indicates that a primordial serine protease gene duplicated early and often during the evolution of mammals to generate a panel of homologous tryptases in each species that differ in their tissue expression, substrate specificities, and physical properties.